Agricultural Research Service (ARS) scientists in California have developed a new raisin grape that dries naturally on the vine, making it unnecessary for growers to cut into the woody “canes” connecting the grapes to the mother vine weeks before harvest.

For mechanically harvested raisin grapes like Thompson Seedless, growers have to cut the canes each summer, usually about 2 weeks before harvest, when the fruit sugar in the grapes has reached a desired level, explains Craig Ledbetter, a geneticist with the ARS Crop Diseases, Pests and Genetics Research Unit in Parlier, California. This cutting helps dry out the grape. The wilted grapes are then shaken off the cut canes mechanically and laid on a continuous paper tray arranged in rows on the ground between the vines. The wilted grapes continue to dry on the ground and turn into raisins, which are then mechanically collected. Other mechanically harvested raisin grapes dry out on cut canes, instead of on the ground, and are then shaken off so that they fall into bins as raisins.

The new “Sunpreme” is the first raisin grape variety that dries on the vine naturally, says Ledbetter. The canes don’t have to be cut with this variety. Sunpreme was bred by retired ARS horticulturalist David Ramming and his colleagues to produce grapes on strong woody canes that begin to wilt naturally in mid-July in California’s San Joaquin Valley, which is a major raisin-production area. Sunpreme’s labor-saving trait of drying naturally on the vine is expected to make it extremely popular with nurseries and grape producers, Ledbetter says.

Based on 10 years of field data, Sunpreme vines yield about 4 tons of raisins per acre, comparable to the yields of other raisin grapes. The vines also grow wherever other raisin grapes grow, and they produce high-quality, medium-sized, green seedless berries. The raisins are slightly larger and fruitier in flavor than the classic Thompson Seedless, Ledbetter says. Pruning of Sunpreme vines back to the short fruiting limbs or “spurs” is much easier than the cane-pruning and tying process required with Thompson Seedless each winter.

ARS is the U.S. Department of Agriculture’s (USDA) principal intramural scientific research agency, and this work supports the USDA goal of promoting international food security.

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